1) Field of the Invention
The present invention relates to an oxide-based superconductor, formed by reaction of a low melting point composition with a solid composition, a process for producing the same and a wire material comprising the same.
2) Related Prior Art
It is known that oxide-based superconductors comprising Tl, Pb, Sr, Ca and Cu have a high critical temperature, and magnetically determined critical current densities of their superconductor grams are high at 77 K in a high magnetic field. Thus, oxide-based superconductors comprising Tl, Pb, Sr, Ca and Cu are promising materials for superconducter wire materials, thin film devices, etc. for use at a liquid nitrogen temperature in a high magnetic field.
Further, it is well known in the process for preparing oxide superconductors to utilize a diffusion reaction between two kinds of substances, which each comprise a combination of oxide superconductor-constituting elements [Spring Annual Conference of Teion Kogaku-Tyodendo Gakkai (Cryogenic-superconductor Society), Lectures, page 98 (1988) and page 121 (1990)].
It has been a problem that the critical transport current densities of bulk materials, wire materials, thin films, etc. prepared from an oxide-based superconductor comprising Tl, Pb, Ba, Sr, Ca and Cu are considerably lower than the above-mentioned magnetically determined critical current densities of crystal grains, add thus it has been difficult to use the oxide-based superconductor in desired products through which a superconducted current is actually to pass. The decrease in the current density of the superconductor members formed therefrom is mainly due to insufficient formation of transport current paths within the members. For example, non-superconductor phases present in the superconductor materials directly inhibit formation of current paths. Furthermore, difference in the orientation of growth of adjacent superconductor crystal grains themselves or disturbances or defects formed at their boundaries give an adverse effect on the electrical characteristics among the crystal grains. In order to realize the distinguished characteristics properly possessed by an oxide super-conductor comprising Tl, Pb, Sr, Ca and Cu such as a high current density in the members formed therefrom, it has been necessary to improve the electric contact among the grains by reducing the non-superconductor phases, increasing the crystal grain sizes (reducing the crystal boundaries), orienting the crystals, and cleaning the crystal boundaries.